Authorities are calling the tainted bottles of water that sickened 12 students at La Mesa Junior High School Thursday an isolated incident following preliminary test results Friday.

The 12 students who were sent to the hospital Thursday after complaining of feeling unwell were released the same day after thorough testing, said Pat Willett, William S. Hart Union High School District.

In all, the 12 students said they drank from eight bottles of water that were dispensed by an on-campus vending machine, Principal Pete Fries said. Two bottles containing the tainted water were recovered.

No students came forward Friday, he said.

Fries discussed the situation with La Mesa students during a school-wide assembly and sent messages home to parents Friday.

But students were still uneasy Friday.

"I'm pretty scared," La Mesa seventh-grader Tasha McDermott said. "You never know what's going to be in the next bottle."

Tasha, 12, said she's been drinking from the school's water fountains, but even that concerns her because she doesn't always know what's in the tap water.

Tasha said she's no longer buying water from the school's vending machines and plans to bring water from home.

The incident began when a La Mesa student purchased a bottle of Aquafina water out of an on-campus vending machine Thursday.The water was reportedly cloudy and smelled like a bleach-like substance.

Several hours after drinking about a third of the bottle, the student experienced stomach cramps.

The school notified the students of the incident and 11 additional students came forward Thursday, saying that they were sickened by Aquafina bottles of water.

They reported symptoms of headaches, stomach aches and nausea.

While an agent with the FBI's Hazmat team initially responded to the school as a precaution, the field testing of the water the students bought is being conducted by Los Angeles County at a county lab.

"At this time, there's no evidence that the product was deliberately tampered with," FBI spokeswoman Laura Eimiller said Friday.

Field tests by a Los Angeles County Fire Department hazardous materials team found no trace of ammonia, chlorine or other contaminants, Eimiller said, adding that further tests were planned at a county lab.

Pepsi Bottling Group, which distributes Aquafina water, collected bottles from the vending machine. There didn't appear to be any adulteration "from a standpoint of the look of the product and the taste," Jeff Dahncke, director of public relations, said.

However, more comprehensive testing will be done, he said.

Pepsi workers visited the school Thursday and removed the school's entire stock of Aquafina water from the vending machines and the school's storage area as a precaution, Fries said.

The empty machines were refilled with a new stock of Aquafina water Friday, Fries said.

Dahncke said the bottles apparently came from a Pepsi plant in Torrance that produced approximately 5,000 cases of Aquafina in a two-day run.

Records on hourly testing during the run revealed no indication of contamination, he said.

Pepsi Bottling Group issued a statement Thursday that said, "There is absolutely no evidence that (Thursday's) incident was caused by anything in our manufacturing process."